Synthetic Approaches to Break the Chemical Shift Degeneracy of Glycans

Influence of Selective Deoxyfluorination on the Molecular Structure of Type-2 N-Acetyllactosamine

N-Acetyllactosamine is a common saccharide motif found in various biologically active glycans. This motif usually works as a backbone for additional modifications and thus significantly influences glycan conformational behavior and biological activity. In this work, we have investigated the type-2 N-acetyllactosamine scaffold using the complete series of its monodeoxyfluorinated analogs. These glycomimetics have been studied by molecular mechanics, quantum mechanics, X-ray crystallography, and various NMR techniques, which have provided a comprehensive and complete insight into the role of individual hydroxyl groups in the conformational behavior and lipophilicity of N-acetyllactosamine.

Nuclear magnetic resonance-based structural elucidation of novel marine glycans and derived oligosaccharides

Marine glycans of defined structures are unique representatives among all kinds of structurally complex glycans endowed with important biological actions. Besides their unique biological properties, these marine sugars also enable advanced structure-activity relationship (SAR) studies given their distinct and defined structures. However, the natural high molecular weights (MWs) of these marine polysaccharides, sometimes even bigger than 100 kDa, pose a problem in many biophysical and analytical studies. Hence, the preparation of low MW oligosaccharides becomes a strategy to overcome the problem. Regardless of the polymeric or oligomeric lengths of these molecules, structural elucidation is mandatory for SAR studies. For this, nuclear magnetic resonance (NMR) spectroscopy plays a pivotal role. Here, we revisit the NMR-based structural elucidation of a series of marine sulfated poly/oligosaccharides discovered in our laboratory within the last 2 years. This set of structures includes the α-glucan extracted from the bivalve Marcia hiantina; the two sulfated galactans extracted from the red alga Botryocladia occidentalis; the fucosylated chondroitin sulfate isolated from the sea cucumber Pentacta pygmaea; the oligosaccharides produced from the fucosylated chondroitin sulfates from this sea cucumber species and from another species, Holothuria floridana; and the sulfated fucan from this later species. Specific 1H and 13C chemical shifts, generated by various 1D and 2D homonuclear and heteronuclear NMR spectra, are exploited as the primary source of information in the structural elucidation of these marine glycans.

Dissecting the Conformational Stability of a Glycan Hairpin

Systematic structural studies of model oligopeptides revealed important aspects of protein folding and offered design principles to access non-natural materials. In the same way, the rules that regulate glycan folding could be established by studying synthetic oligosaccharide models. However, their analysis is often limited due to the synthetic and analytical complexity. By utilizing a glycan capable of spontaneously folding into a hairpin conformation as a model system, we investigated the factors that contribute to its conformational stability in aqueous solution. The modular design of the hairpin model featured a trisaccharide turn unit and two β-1,4-oligoglucoside stacking strands that allowed for systematic chemical modifications of the glycan sequence, including the introduction of NMR labels and staples. Nuclear magnetic resonance assisted by molecular dynamics simulations revealed that stereoelectronic effects and multiple glycan-glycan interactions are the major determinants of folding stabilization. Chemical modifications in the glycan primary sequence (e.g., strand elongation) can be employed to fine-tune the rigidity of structural motifs distant from the modification sites. These results could inspire the design of other glycan architectures, with implications in glycobiology and material sciences.